The present invention relates to a method for detecting overpressure inside a compartment associated with a gas turbine nacelle, including so-called “fire zones” formed by a fan nacelle or a core nacelle surrounding a jet engine in an aircraft.
Gas turbines may be provided with a nacelle. In aerospace applications in particular, it is common for a gas turbine engine to be fitted with one or more such nacelles.
By way of example, FIG. 1 shows a simplified cross-sectional overview of a typical turbofan engine 1 in a civil aircraft.
The engine 1 is supported inside two nacelles: an inner, core nacelle 2 which surrounds the engine core 1a, and an outer, hollow-wall fan nacelle (or “fancase”) 3 which surrounds the by-pass fan 1b. The outer surface of the core nacelle 2 and the inner surface of the fan nacelle 3 together define an annular fan discharge passageway 4 for the respective bypass jet from the bypass fan 1b. 
In order to limit the spread of a fire throughout the respective nacelle structure, each nacelle in a gas turbine engine will typically be designed so that it forms one or more associated fire containment compartments, or “fire zones”.
FIG. 1 shows a typical configuration of fire zones in a civil aerospace application.
Thus, the fan nacelle 3 includes a single fire zone, conventionally designated as “Zone 1”, in the form of a fancase compartment 5 enclosed between respective fireproof bulkheads 6, 7 inside the hollow-wall of the fan nacelle 3.
In the case of the core nacelle 2 there are two fire zones, conventionally designated “Zone 2” and “Zone 3”, which are in the form of first and second core compartments 8,9 defined between the inner surface of the nacelle 2 and the outer surface of the engine core 1a. The core compartments 8, 9 are separated from one another by a fireproof bulkhead 10 extending between the inner surface of the nacelle 2 and the outer surface of the engine core 1a. 
Conventionally, each fire zone is cooled and ventilated by air being passed around the engine and vented overboard. Thus, fan compartment 5 includes an inlet 5a (represented symbolically by a flow direction arrow in FIG. 1) for receiving air from the atmosphere and a respective outlet 5b for venting air to the atmosphere, the first core compartment 8 includes an inlet 8a for receiving air from the fan discharge passageway 4 and a respective outlet 8b for venting air back into the fan discharge passageway 4, and the second compartment similarly includes an inlet 9a which receives air from the fan discharge passageway and an outlet 9b for venting this air to atmosphere.
Cooling and venting of the fire zones helps continually purge the zone of flammable vapour and also regulates the temperature inside the zone to help reduce the possibility of fire ignition.
In addition to the provision and arrangement of fire zones within the nacelle structure, each nacelle will additionally be protected by a fire warning system, being either a dedicated nacelle system or, particularly in the case of Zone 2 and Zone 3, forming part of a larger Fire Detection System (FDS) for the engine 1.
The respective fire warning system typically utilises a network of local temperature sensors around the nacelle to detect a fire-critical overheating within the nacelle structure. In the case of the FDS, this will normally trigger a fire alarm message in the cockpit, and the flight manual will then instruct the aircrew to take suitable “reversionary” action, for example throttling back on the engine and possibly shutting down one or more auxiliary engine systems, in order to limit the temperatures within the nacelle as far as possible.